The present invention relates to a radar system and a car radar system and more particularly to a car radar system for detecting the range or range rate between an own car and a target such as a precedent car or an obstacle.
A radar using radio waves such as millimeter waves, since the propagation loss of a radio beam is little even in bad weather including rain or mist and radio waves reach faraway, has been widely used in the fields of flight control and weather observation. Further, in recent years, in the field of prevention of a traffic accident and security of cars, a millimeter wave radar for measuring the distance from the car ahead and range rate with a precedent car has been researched, developed, and commercialized.
There are some kinds of high frequency circuits of a millimeter wave radar available, and particularly an MMIC (microwave monolithic IC) having the characteristics of small size, low cost, and high reliability is useful for cars.
The MMIC has a built-in FET (p-HEMT) with an AlGaAs/InGaAs double hetero structure having excellent high frequency characteristics, and in order to drive FET (Field Effect Transistor), it is necessary to control a plurality of power supplies of a drain power supply and a gate power supply.
An electron control unit having a plurality of power supplies is disclosed in Japanese Application Patent Laid-open Publication No. 2001-312318. The electron control unit, as shown in FIG. 12, has a plurality of power supplies of a 5V power supply 201 and a 3.3V power supply 202 and among them, the 3.3V power supply 202 is used as a power supply for a microcomputer 101 and a reset controller 102 for the microcomputer 101.
To prevent the microcomputer 101 from an unstable operation at the time of the start and shut-off of the power supply, the start and shut-off sequence as shown in FIG. 13 is realized. Namely, at the time of the start, the 3.3V power supply 202 rises, and then the 5V power supply circuit 201 rises, and after the delay time Treset elapses from the point of time where the 5V power supply reaches the reset level or higher, the microcomputer 101 is released from reset. Further, at the time of shut-off, the 5V power supply 201 falls earlier than the 3.3V power supply 202, and the microcomputer 101 is put into the reset state. Thereby, the indefinite operation area of the microcomputer can be eliminated.
The FET built in the MMIC for a millimeter wave radar changes the path width (the cross sectional area of the channel) of electrons moving from the supply to the drain by the gate voltage to be reversely biased, thereby controls the drain current. And so, if the drain power supply is applied in a state that no gate power supply is applied, the current to be supplied to the FET is not controlled, thus the FET may be broken down.
Therefore, in control of the two power supplies of the drain power supply and gate power supply of the FET, when the drain power supply is applied, the gate power supply must be applied always.
Further, even if the drain power supply of the FET is shut off, thereafter, when the gate power supply is shut off in a state that some charge remains in the drain circuit, an excess current flows in the FET by the residual charge, thus the FET may be broken down.
Further, when the drain power supply is applied in a state that the gate power supply and drain power supply of the FET are respectively at a potential beyond the specified ranges, the current to be supplied to the FET increases beyond the control area, thus the FET may be broken down.
In this respect, the prior art disclosed in Japanese Application Patent Laid-open Publication No. 2001-312318 mentioned above is unsolved in the following point. It will be explained hereunder by referring to FIG. 14.
In the constitution shown in FIG. 14, at the later stage of the 5V power supply, a plurality of power supplies are additionally installed. Namely, a (−)5V power supply 203 is operated by the 5V power supply 201, there exist circuit elements of both power supplies which are controlled by the 5V power supply 201 and the (−)5V power supply 203.
In such a circuit constitution, it is not sufficient only to eliminate the indefinite operation area of the microcomputer by controlling the operations of the 3.3V and 5V power supplies. Namely, it is necessary to provide a circuit constitution for protecting the circuit elements of both power supplies of the 5V power supply 201 and the (−)5V power supply 203 operating according to the 5V power supply 201. The prior art disclosed in Japanese Application Patent Laid-open Publication No. 2001-312318 cannot protect the FET of a millimeter wave radar using the aforementioned MMIC.